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Thursday, September 15, 2016

Experimental Search for Quantum Gravity 2016

Research in quantum gravity is quite a challenge since we neither have a theory nor data. But some of us like a challenge.

So far, most effort in the field has gone into using requirements of mathematical consistency to construct a theory. It is impossible of course to construct a theory based on mathematical consistency alone, because we can never prove our assumptions to be true. All we know is that the assumptions give rise to good predictions in the regime where we’ve tested them. Without assumptions, no proof. Still, you may hope that mathematical consistency tells you where to look for observational evidence.

But in the second half of the 20th century, theorists have used the weakness of gravity as an excuse to not think about how to experimentally test quantum gravity at all. This isn’t merely a sign of laziness, it’s back to the days when philosophers believed they could find out how nature works by introspection. Just that now many theoretical physicists believe mathematical introspection is science. Particularly disturbing to me is how frequently I speak with students or young postdocs who have never even given thought to the question what makes a theory scientific. That’s one of the reasons the disconnect between physics and philosophy worries me.

In any case, the cure clearly isn’t more philosophy, but more phenomenology. The effects of quantum gravity aren’t necessarily entirely out of experimental reach. Gravity isn’t generally a weak force, not in the same way that, for example, the weak nuclear force is weak. That’s because the effects of gravity get stronger with the amount of mass (or energy) that exerts the force. Indeed, this property of the gravitational force is the very reason why it’s so hard to quantize.

Quantum gravitational effects hence were strong in the early universe, they are strong inside black holes, and they can be non-negligible for massive objects that have pronounced quantum properties. Furthermore, the theory of quantum gravity can be expected to give rise to deviations from general relativity or the symmetries of the standard model, which can have consequences that are observable even at low energies.

The often repeated argument that we’d need to reach enormously high energies – close by the Planck energy, 16 orders of magnitude higher than LHC energies – is simply wrong. Physics is full with examples of short-distance phenomena that give rise to effects at longer distances, such as atoms causing Brownian motion, or quantum electrodynamics allowing stable atoms to begin with.

I have spent the last 10 years or so studying the prospects to find experimental evidence for quantum gravity. Absent a fully-developed theory we work with models to quantify effects that could be signals of quantum gravity, and aim to test these models with data. The development of such models is relevant to identify promising experiments to begin with.

We’ll hear about the prospects of finding evidence for quantum gravity in the CMB (Bianchi, Krauss, Vennin) and in quantum oscillators (Paternostro). We have a lecture about the interface between gravity and quantum physics, both on long and short distances (Fuentes), and a talk on how to look for moduli and axion fields that are generic consequences of string theory (Conlon). Of course we’ll also cover Loop Quantum Cosmology (Barrau), asymptotically safe gravity (Eichhorn), and causal sets (Glaser). We’re super up-to-date by having a talk about constraints from the LIGO gravitational wave-measurements on deviations from general relativity (Yunes), and several of the usual suspects speaking about deviations from Lorentz-invariance (Mattingly), Planck stars (Rovelli, Vidotto), vacuum dispersion (Giovanni), and dimensional reduction (Magueijo). There’s neutrino physics (Paes), a talk about what the cosmological constant can tell us about new physics (Afshordi), and, and, and!

But the best is I’m not telling you this to depress you because you can’t be with us, but because our IT guys still tell me we’ll both record the talks and livestream them (to the extent that the speakers consent of course). I’ll share the URL with you here once everything is set up, so stay tuned.

Detection of gravitons would be direct confirmation of the quantization of the gravitational field. There's a fascinating paper by Freeman Dyson from 2012: "Is a Graviton Detectable?", that discusses this very idea: https://publications.ias.edu/sites/default/files/poincare2012.pdf

Unfortunately, the paper concludes there's no practical way to detect gravitons directly, even en-masse, primarily due to their vanishingly small interaction cross-section with ordinary matter. Dyson refers, towards the end of the paper to the Gertsenshtein process, which appears to hold out the most promise, though still way insufficient for viable detection, even from anticipated astrophysical sources of this process, like pulsars.

The live video feeds of the talks at the Frankfurt Institute for Advanced Studies should be very enlightening.

Bee, sorry, I didn't mean to imply that. I'm actually quite optimistic that a signature of quantum gravity will show up in the high precision experiments that will be discussed at the Frankfurt Institute Symposium next week.

Hey sabine I am an undergraduate math major and I need some career advice. I deeply enjoy mathematics both for its own sake and for the crazy structures it can bring about, but what i love more is the idea of these structures popping up in the physical world. I feel a draw toward discovering these things in the real world and they pop up in theories of QG all the time. I also love reimannian geometry and GR; in that sense I feel compelled to work on issues involved with QG, but being so mathematical I feel like there is no room for me in physics right now. I asked one of my physics profs about this and he said just to go into numerical analysis for bombs or rockets and ignore this stuff. Obviously thats not going to happen. I am not sure what to do in this scenario

Btw the book that got me hooked on QG was "Quantum Gravity: Mathematical Models and Experimental Bounds" its a organized collection of papers from a workshop in Germany, it has some pretty amazing stuff. Also my name is Nathan and if you read and reply to this, thank you for reading my novel of a comment.

First, if you like differential geometry, you could do the easy thing and go into GR (classical), which is presently blooming especially in the gravitational wave regime (guess why). Yes, much of it is numerical, but it's close enough to what you seem to be interested in. And what most of us do anyway is that we have what my supervisor used to call 'butter-and-bread' work (that publishes and pays the bills) and some pet topic on the side.

Having said that, you write you want to 'discover these things in the real world' but then you go on to say that they pop up in theories of QG. You might want to do some thinking about how you know that these theories actually have something to do with the real world.

As long as you're below the age of, say, 35, it isn't difficult to find some temporary contract doing something with quantum gravity because the system is presently built around exploiting young people and there's money to make in that. Problem is, there are no positions to land on later. So if you go into the field, you have keep in mind that with very high probability you'll have to change your profession in your upper 30s. Best,

Would you agree that progress in this area has been slow because the various quantum gravity approaches being researched make no new predictions that can be verified by experiment today? Doesn't matter how much beauty a model has, if nature doesn't agree with it then it is not much use. Shouldn't all models proposed have an obligation to ask nature if their model is correct? Look what happened at string theory for eg.

I will be watching some of the lectures on video. Thanks for the reminder.

I fully realize that to most physicists, E = hv is just as true in gravity as it is in electromagnetism, but there exists no experiment to show that this relation also holds for gravity. What if gravity is not quantized at all? Are there any experiments that could detect that?

Look at a hydrogen atom (in the first excited state). Quantum Mechanics predicts the emission of a graviton once every 10^38 seconds or so. With an un-quantized gravity, the atom would instead gradually radiate at 10^-37 eV per second. So in that case having the atom radiate gravitational radiation constantly and classically would result in no measurable changes to physics.

Looking at nuclear systems one can predict much larger classical radiation - about an eV per week in some nuclei it seems (As a simple estimate use the Eddington gravitational wave power formula on moving nucleons). This is still a very small amount of energy on the nuclear scale, but perhaps more amenable to experiment. Looking for (and likely not finding) the effects of classical radiation emission and absorption in nuclei - using bulk matter to scale the effects to measurable levels - might help to prove whether or not gravity is quantized.

Thanks - I had not looked at the Schrödinger-Newton equation much before. I am reading the "Is Quantum Gravity Necessary" paper by Carlip now. http://arxiv.org/abs/0803.3456.

One difference between those papers and what I was talking about is that there seems to be a general agreement that the gravitational field couples to the 'expectation value' . There is also the possibility that GR couples to some 'real instantaneous' position - (e.g. - emergent QM), in which case the experiments become easier, as using expectation values is like a motion blur.

Well, no, most in the community think it does not couple to the expectation value, that's just an approximation. Yeah, you can think of other options, been there done that. Try to write down a consistent theory, publish it, and we can talk.

What requires a black hole to have internal volume and a singularity? Neither are described by physics. Both are inconsistent with LIGO event GW150914. Black holes (BHs) massing 30 or a billion sols may contain mass within externally-viewed volume or a (2 + L_P)-dimensional event horizon shell (L_P = Planck length)). Mass distribution moments of inertia are the "same" for both cases,

The broadcast for the first talk was stopped because the speaker hadn't signed the consent form. Sorry about this. The other speakers today have all signed it, so there should be no more interruptions. I will ask about the brightness. Best,